Histone-Like Nucleoid Structuring Protein Modulates the Fitness of tet(X4)-Bearing IncX1 Plasmids in Gram-Negative Bacteria

Abstract

The emergence of plasmid-mediated tigecycline resistance gene <i>tet</i>(X4) poses a challenging threat to public health. Based on the analysis of <i>tet</i>(X4)-positive plasmids in the NCBI database, we found that the IncX1-type plasmid is one of the most common vectors for spreading <i>tet</i>(X4) gene, but the mechanisms by which these plasmids adapt to host bacteria and maintain the persistence of antibiotic resistance genes (ARGs) remain unclear. Herein, we investigated the underlying mechanisms of how host bacteria modulate the fitness cost of IncX1 plasmids carrying <i>tet</i>(X4) gene. Interestingly, we found that the <i>tet</i>(X4)-bearing IncX1 plasmids encoding H-NS protein imposed low or no fitness cost in <i>Escherichia coli</i> and <i>Klebsiella pneumoniae</i>; instead, they partially promoted the virulence and biofilm formation in host bacteria. Regression analysis revealed that the expression of <i>hns</i> gene in plasmids was positively linked to the relative fitness of host bacteria. Furthermore, when pCE2::<i>hns</i> was introduced, the fitness of <i>tet</i>(X4)-positive IncX1 plasmid pRF55-1 without <i>hns</i> gene was significantly improved, indicating that <i>hns</i> mediates the improvement of fitness. Finally, we showed that the expression of <i>hns</i> gene is negatively correlated with the expression of <i>tet</i>(X4) gene, suggesting that the regulatory effect of H-NS on adaptability may be attributed to its inhibitory effect on the expression of ARGs. Together, our findings suggest the important role of plasmid-encoded H-NS protein in modulating the fitness of <i>tet</i>(X4)-bearing IncX1 plasmids, which shed new insight into the dissemination of <i>tet</i>(X4) gene in a biological environment.